Casting Mould for Casting Steel Melt

ABSTRACT

A casting mould for casting steel melt into a continuously drawn strand is provided herein. A surface texture is formed on at least one inner surface of the casting mould facing towards the melt to be cast. The surface texture extends at least over region of the casting mould which, during operation, is wetted with slag floating on the melt that has been poured into the casting mould. The casting mould allows optimum solidification behaviour in the region of the casting mould that is critical in terms of the risk of crack formation. The surface texture is designed as a closed structure with self-contained, randomly distributed indentations.

The invention relates to a casting mould for casting steel melt into astrand being continuously drawn, wherein on at least one of the innersurfaces of the casting mould facing towards the melt to be cast asurface texture is formed, which extends at least over the region of themould, which, during operation, is wetted with slag floating on the meltthat has been poured into the mould.

In continuous casting, steel melt is poured from a casting ladle into adistributor, also known as “tundish”, provided as a buffer and, whereappropriate, to distribute the melt into several strands, and istransported from there into the respective casting mould by means of adip tube. The pouring direction corresponds to the force of gravityhere.

The strand is formed in the casting mould. Upon contact with the coolinner surfaces of the casting mould the melt begins to solidify suchthat the strand coming out of the casting mould in a vertical directionhas a thin shell of solidified steel on its outer surfaces, the shellenclosing the still liquid melt inside the strand.

After emersion from the mould, the strand is redirected supported byrolls in a so-called ‘casting bow’ in a horizontal direction of flow.Systematically controlled cooling takes place in the region of thecasting bow in order to effect a controlled solidification of thestrand. From the strand being completely solidified and discharged in ahorizontal direction slabs are then separated, which and conveyed forfurther processing.

Casting powder is scattered in the mould on the free surface of the meltto form slag. The slag covers the melt and prevents the melt fromreacting with the surrounding atmosphere in the region of the so-called‘meniscus’. At the same time the slag binds impurities ascending in themelt and acts as a lubricant between the solidifying shell of the steelstrand and the mould. Alternatively, there are casting methods in whichpre-molten casting powder is fed or in which the casting method usesso-called ‘casting oils’ i.e. liquid casting media, instead of castingpowder. The latter technique is applied particularly in billet orcircular continuous casting. The mould is generally moved in anoscillating manner in order to prevent the steel from sticking to thecooled walls of the mould and to support the discharge of the strandfrom the mould.

Continuous casting moulds can be composed of mould plates or designed asone individual piece. The internal sides of continuous casting mouldsare generally made of copper. To improve their resistance to wear andtear the inner surfaces of said casting moulds that come into contactwith the forming strand can be covered with a nickel coating (EP 0 125509 B1). However, the nickel coating results in a significant reductionin heat flow. For this reason it is generally applied only at a certaindistance from that upper edge of the casting mould, which is assigned tothe distributor of the continuous casting plant.

Regardless of whether or not the inner surfaces of casting moulds arenickel coated, in the continuous casting method the steel melt coolsparticularly rapidly especially in the region of the meniscus. This canlead to surface defects in the case of sensitive steel grades due to theinternal stresses that occur during the cooling process.

This issue was already addressed in EP 1 099 496 B1. Reference is madethere to the publication ‘Über den Zusammenhang zwischenAnfangserstarrung and Beschaffenheit der Strangoberfräche beiperitektisch erstarrenden Stählen’ (Postdoctoral thesis by M. M. Wolf,Forch 2002, pages 61-64) according to which in particular the thermalflow through the mould wall in the region of the molten metal levelthereof plays a crucial role in terms of the absence of cracks in thestrand shell. If the heat flow is too great, this results in anincreased risk of cracking. In order to increase the thermal flowbetween the forming strand shell and the inner surface of the mould, EP1 099 496 B1 suggests reducing the thermal resistance in the region ofthe meniscus by roughening the mould surface. In this way, the strandshell forming in the mould should remain thinner for longer and bepressed evenly against the copper plate of the continuous casting mouldby the ferrostatic pressure rising with increasing distance from thelevel of the molten metal. The surface of the mould is roughened in theprocess such that the machining depth of the roughness of the innersurface of the mould decreases in the casting direction such that agradual transition is achieved from rough to smooth section of the mouldand thus also a gradual transition from restricted to unrestrictedthermal flow. One advantage considered there is that the macrostructureof the inner surface of the mould can be achieved by methods known perse such as shot blast texturing (SBT), electric discharge texturing(EDT), electron beam texturing (EBT), laser texturing (LT) or by aperforated texture (GLT) or using other methods.

In the light of the above-mentioned prior art, the object of theinvention was to create a casting mould where with simple means anoptimum solidification behaviour is guaranteed in the region of thecasting mould that is critical in terms of the risk of crack formation.

This problem is solved according to the invention by a casting moulddesigned according to Claim 1.

Advantageous embodiments of the invention are stated in the dependentclaims and explained in detail below along with the general inventiveconcept.

In a casting mould according to the invention for casting steel meltinto a strand being continuously drawn, a surface texture is formed onat least one of the inner surfaces of the casting mould facing towardsthe melt to be cast in accordance with the prior art explained above.The surface texture extends at least over the region of the mould,which, during operation, is wetted with slag floating on the melt pouredinto the mould.

According to the invention, said surface texture is now designed as aclosed structure with indentations which are completely bordered andrandomly distributed indentations.

The structure provided as surface texture according to the invention andformed from entirely defined indentations reduces the transfer of heatbetween mould and liquid melt. Some of the solidifying slag covers theindentations on the random surface structure and sticks there unlike inthe case of open surface structures. Thus, the slag adhering to theinner surface of the mould acts as heat insulation which prevents directcontact of the melt with the inner surface. Said insulating effect ofthe slag layer leads to a lower, and over the breadth of the mould, to amore uniform heat supply in the region of the meniscus. As a consequenceof the overall reduced and more uniform heat supply, fewer internalstresses occur in the strand shell during the cooling process when usinga structured mould surface according to the invention compared with aconventional mould surface. Consequently, the risk of surface defectsforming is reduced. If casting oils are used, the surface texturedescribed here is wetted. The oil layer then adhering in theindentations also acts as thermal insulation.

An open surface and roughness structure created using the methodreferred to in EP 1 099 496 B1, for example, or by shot blasting orsimilar methods, in which the respective indentations overlap andaccordingly are not defined from each other, but merge together, obtainsits roughness from elevations in the material, which occur due to ashifting of the mould material. The closed surface structure providedaccording to the invention is, however, characterised by indentationsand cavities that are not connected. It turns out that said closed, andaccording to the invention randomly distributed, indentations ensurebetter slag adhesion and prevent slag run off.

In addition to the topographical appearance that occurs in this manner,the mean roughness index Ra and the mean roughness depth Rz areimportant in terms of designating said surface structure. Both the meanroughness index Ra and the mean roughness depth Rz must be determined inaccordance with DIN EN ISO 4287. In the case of a surface structureaccording to the invention, the ideal mean roughness index Ra is between10 m and 50 μm and the mean roughness depth Rz between 80 μm and 250 μm.Mean roughness values and mean roughness depths in said value rangesresult in a maximum reduction of surface defects and stable processreliability. This applies particularly if the mean roughness index Ra isbetween 10 μm and 50 μm, in particular between 15 μm and 50 μm.

Optimum adhesion of the slag on the surface texture is produced if themaximum depth of indentations of the surface texture is 500 μm. Theindentations should be at least 5 μm deep in order to reliably achievethe pursued roughness.

Casting moulds of the type referred to here are normally made of anon-ferrous metal alloy, which is generally cooled on the side facingaway from the melt. The mould cross-section can be designed as square orrounded. In order to produce strands of varying widths when usingrectangular or square moulds, at least one of the plates defining thenarrow sides of the mould opening can be adjustable in the widthdirection (EP 0 985 471 A1 ).

The surface structure provided according to the invention is provided onat least one of the inner surfaces defining the casting mould openings.Naturally, this includes the option to form a corresponding surfacestructure on all or at least opposite inner surfaces of the castingmould. Also in the case of width-adjustable casting moulds, the surfacetexture structured according to the invention should be present on atleast one of the inner surfaces. The region, which is covered duringadjustment of the side of the mould moved relative to said innersurface, can remain free of the surface structure according to theinvention if this is advantageous in terms of sealing the corner regionsin which the surfaces defining the mould opening touch. Accordingly, inthe case of a casting mould, the thickness or width of which can beadjusted by displacing at least one of the sides thereof, the surfacetexture extends over the width of the inner surface provided with saidsurface texture, by means of which the inner surface comes into contactwith the melt to be cast if the smallest thickness or width of thecasting mould is configured.

The surface texture structured according to the invention should extendat least over the region of the respective inner surface of the castingmould, which is wetted during casting operations by the slag coveringthe meniscus. It has proven useful in the case of casting moulds usedtoday if the surface texture extends over an area which, measured in thecasting direction, begins at a distance of at least 10 mm below theupper mould edge and ends at a maximum distance of 600 mm.

In the event that the inner surface provided with the surface structureis covered with a layer of nickel over a section beginning at a distancefrom the upper edge of the mould, it has proven particularlyadvantageous in terms of reducing surface defects in the casting strandif the surface texture designed according to the invention overlaps theedge region assigned to the upper edge of the mould. In practice,overlapping areas have proven useful here, which, measured in thecasting direction, are at least 50 mm. The overlapping of the surfacetexture according to the invention with the nickel coating prevents anabrupt break in thermal conductivity in the transition zone between thenon-coated to the nickel-coated section of the respective inner surface.

The structure according to the invention of the surface texture providedon the respective mould surface can be introduced into the surface byembossing (pressure) or by strike or impact momentum, using needles, forexample. The structure is introduced by deforming the mould surfacewithout removing material in the process. The cold work hardeningeffected as a result of the striking or pressing strain on therespective mould inner surface can contribute towards a longer usefullife of the mould.

If a stamping method is used, a negative of the structure to be producedis applied to a matrix, a sphere or a roller. Said negative is then usedto apply the surface structure to the mould, depending on pressure andtool surface. If the structure is produced using a method based onstrike or impact momentum, the structure defined according to theinvention is produced by a tool striking with high momentum. So-called‘needles’ with which specific surface roughnesses can be generated aresuitable for this, such as in DE 199 07 827 A1, for example.

The invention is explained in greater detail below using drawingsrelating to an embodiment. Each of the figures represents a schematicview.

FIG. 1 shows a side view of a strand casting plant;

FIG. 2 shows a longitudinal section of a casting mould used in thestrand casting plant according to FIG. 1;

FIG. 3 shows a perspective view enlarged 7.5 times of section of asurface texture provided according to the invention.

To cast a steel melt M into a strand S in the strand casting plant 1shown in FIG. 1 and constructed in a manner known per se, the steel meltM is transported in a ladle 2 to a distributor 3 and poured into thedistributor 3 by means of a ladle shroud 4. To a base outlet of thedistributor 3 a further vertically aligned dip tube 5 is connected,which can be closed and controlled by a stopper 6.

When the dip tube 5 is open, the steel melt M flows into a casting mould7, which is composed of cooled plates 8, 9, 10, 11, which are made of anon-ferrous metal or a non-ferrous metal alloy. Preferably, copper orcopper alloys are used. The casting mould 7 has an opening cross-sectionthat is substantially rectangular when viewed from above. The long sidesof said cross-section are respectively delimited by a wide mould plate8, 9 and the short sides respectively delimited by a narrow mould plate10, 11.

On their inner surfaces 13 respectively assigned to the casting mouldopening 12, the mould plates 8-11 can often be covered with a nickellayer 14, which, measured in the vertically aligned direction of flow Fof the steel melt M, begins at a variable distance from the upper edge15 of the casting mould 7 assigned to the distributor 3. The distance A1is 300 mm in this case, but can be configured as generally variable. Arectangular mould with a nickel layer is used as an example here.However, other mould shapes with different coatings are also possible.

The strand S forming in the casting mould 7 from the steel melt M comesout of the casting mould 7 in a vertical direction of flow F and isguided in a horizontal direction Fh by means of a casting bow 16. In theregion of the casting bow 16 the strand is guided by rollers 19, 20.Intensive cooling takes place at the same time such that the strand Shas completely solidified to the greatest possible extent by the time itreaches the end of the casting bow 16 and can be conveyed for furtherprocessing.

A surface texture 22 is configured on the inner surfaces 13 of the mouldplates 8-11 defining the mould opening 12 in a section 21 assigned tothe upper edge of the mould 15. In this embodiment, the surface texture22 begins in the direction of flow F at a distance A2 of 10 mm and endsat a distance A3 of 400 mm from the upper edge of the mould 15.Accordingly, the surface texture 22 overlaps the nickel layer 14 in anoverlapping region U over a length measured in the direction of flow Fof 100 mm. The surface texture can generally also be introduced up to adistance A3 of 600 mm as seen from the upper edge of the mould 15. Inthe section covered by the surface texture 22, the slag K floating atthe meniscus of the melt M to be cast during casting operations wets theinner surface 13 of the copper plates 8-11.

The surface texture 22 is formed by a plurality of indentations 23,which are each completely enclosed by a partition wall 24. Eachpartition wall 24 defines two adjacently arranged indentations 23. Theindentations 23 can be formed as individual hole-like impressions with asubstantially round opening cross-section or from several suchimpressions merging together, which are then in turn bordered by aself-contained partition wall 24 encircling the respective indentation23. Material ridging, which is produced when using the shot-blastingmethod, for example, is undesirable in this structure as said ridging isworn down by the strand shell. This would result in a degeneration ofthe structure reducing the roughness properties. Indentations are ratherintroduced into the mould material in order to achieve cold workhardening and maintain the surface structure. In the mould plates 8, 9defining the mould opening 12 on the long sides thereof, the width B ofthe surface texture 22 is restricted to the narrowest region, which, ifthe mould plates 10, 11 defining the mould opening 12 on the short sidesthereof are moved, is not covered by the copper plates 10, 11.

The indentations 23 being up to 500 μm deep have been produced byneedles, using a standard needle device, that is not shown here. Theneedles in the needle device have been driven into the inner surfaceusing high force and have compacted the material with which they havecome into contact thus forming the respective indentation 23. Nomaterial abrasion occurred. In order to maintain the structurecomprising indentations 23 and partition walls 24 shown in FIG. 3, thefollowing parameters were set:

-   -   distance between needle device housing and surface to be        machined,    -   feed speed and feed direction,    -   movement pattern of the needle device housing/needle device and    -   force with which the needles strike the surface to be machined.

The mean roughness depth Rz and mean roughness index Ra for two surfacetextures produced in this way inside and outside the overlapping regionof surface texture 22 and nickel layer 14 of the inner surfaces 13 areshown in Table 1.

TABLE 1 Sample Sample roughness roughness Nickel- from non- from platedRoughness nickel- nickel- Example mould? parameter plated area platedarea 1 No Ra  26.42 μm — Rz 120.34 μm — 2 Yes Ra  22.31 μm 16.99 μm Rz121.20 μm 95.39 μm 3 Yes Ra  41.28 μm 18.91 μm Rz 187.33 μm 93.66 μm

REFERENCE SIGNS

1 Continuous casting plant

2 Ladle

3 Distributor (Tundish)

4 Ladle shroud

5 Dip tube

6 Stopper

7 Casting mould

8-11 Copper plates

12 Casting mould opening

13 Inner surfaces of casting mould 7

14 Nickel layer

15 Upper edge of mould

16 Casting bow

19,20 Rollers

21 Section of the inner surfaces 13

22 Surface texture

23 Indentations

24 Partition wall

A1-A3 Distances, measured in direction of flow F

B Width of section of the inner surface 13 provided with the surfacetexture

F Direction of flow of steel melt M in casting mould 7

Fh Horizontal direction of flow

K Slag

M Melt

S Strand

Ü Overlapping region

1. A casting mould for casting steel melt into a continuously drawnstrand wherein a surface texture is formed on at least one innersurfaces of the casting mould facing towards the melt to be cast, saidsurface texture extending at least over a region of the casting mould,which, during operation, is wetted with slag floating on the melt thathas been poured into the casting mould, wherein the surface texture isdesigned as a closed structure with completely bordered, randomlydistributed indentations. 2.The casting mould according to claim 1,wherein the surface texture extends over an area, which, measured in acasting direction, begins at a distance of at least 10 mm below an upperedge of the mould and ends at a maximum distance of 600 mm.
 3. Thecasting mould according to claim 1, wherein a maximum depth ofindentations of the surface texture is 500 μm.
 4. The casting mouldaccording to claim 1, wherein a mean roughness index of the surfacetexture is between 10 μm and 50 μm.
 5. The casting mould according toclaim 1, wherein a mean roughness depth of the surface texture isbetween 80 μm and 250 μm.
 6. The casting mould according to claim 1,wherein the casting mould has a square or rounded opening cross-sectionand wherein the surface texture is configured on at least one of theinner surfaces of the casting mould, which defines the openingcross-section on a long side thereof.
 7. The casting mould according toclaim 6, wherein the casting mould is width-adjustable by moving a shortside thereof and wherein the surface texture extends over a width of theinner surface provided with said surface texture, by means of which theinner surface comes into contact with the melt to be cast, if thecasting mould is set to the smallest width.
 8. The casting mouldaccording to claim 1, wherein the surface texture is introduced byimpacting the respective inner surface of the casting mould.
 9. Thecasting mould according to claim 8, wherein the surface texture isintroduced into the inner surface using needles.
 10. The casting mouldaccording to claim 8, wherein the surface texture is embossed into therespective inner surface of the casting mould.
 11. The casting mouldaccording to claim 1, wherein the inner surface provided with thesurface texture is covered with a layer of nickel over a sectionbeginning at a distance from an upper edge of the mould and wherein thesurface texture overlaps an edge region of the nickel layer assigned tothe upper edge of the mould.
 12. The casting mould according to claim11, wherein the surface texture overlaps the nickel layer measured in acasting direction by at least 50 mm.